In the lab where I was a graduate student, the IBI electroeluter was
referred to as the "electro-diluter". We rarely got yields above 40% with
it, and often more like 10-20%. We tried many of the same things that you
tried, with no luck. One thing you might try is making sure that all of
the little slots have high salt solution in them, even those that don't
have a DNA slice in them. I seemed to recall that this was important,
although my memory is a little vague.
If yields are important, I would think about another way of recovering
your DNA from the gel.